The autofluorescent serotonin analog, 5,7 dihydroxytryptamine, has been used to identify living dopaminergic neurons derived from the mesencephalon. Midbrains of embryonic day 13 rats were grown in vitro for three weeks, incubated with micromolar amounts of 5,7 dihydroxytryptamine and examined by fluorescence microscopy. When excited by ultraviolet light (wavelength max. 365nm) a small proportion of the neurons displayed a weak, but well defined, blue-violet fluorescence. The neurons so identified were photographed and marked and subsequently fixed in aldehydes to be processed for aldehyde- induced (Faglu) catecholamine fluroescence or tyrosine hydroxylase immunohistochemistry. Recovery of the identified neurons showed that 100% displayed catecholamine fluorescence and over 80% were immunoreactive for tyrosine hydroxylase. These experiments demonstrate that uptake of the fluorescent serotonin analog 5,7 dihydroxytryptamine can be used to identify living dopaminergic neurons derived from the midbrain. This approach will now allow focussed single cell analyses which otherwise would be at best most difficult since dopaminergic neurons constitute one of the rarest transmitter phenotypes in the CNS.